Error with `import_lato()` on read only machine

[vidonne]

The package fails a test on r-devel-linux-x86_64-debian-gcc as the import_lato() function tries to install the font on a read-only machine.

Fix:

• [x] Change lato_registered to also check installation
• [x] Add a test to font_family in theme_unhcr with a fallback to "sans"
• [x] Add a test in update_geom_font